The manufacture of semiconductor packages typically involves the use of substrates such as semiconductor leadframes for mounting integrated circuit chips. Leadframes are commonly made from reactive metals such as iron alloy or copper alloy. Leadframes made from copper alloy are sometimes preferred over iron alloy leadframes due to factors such as better heat dissipation, ease of processing and cost. Nonetheless, the use of copper alloy also has its disadvantages, such as its tendency to oxidize when it reacts with oxygen to produce copper oxide when exposed to oxygen in the air at high temperatures. Such oxidation results in oxygen forming weak bonds with the atoms at the leadframe surface, and a layer of brittle and/or poorly adhering oxides. Thus, oxidation introduces reliability problems for microelectronic packages.
The problem of oxidation is particularly acute during wire bonding in a typical semiconductor packaging process, wherein conductive bonding wires are bonded to contact surfaces on a semiconductor die and a leadframe to establish electrical connections therebetween. This bonding is commonly done by using an ultrasonic transducer to generate mechanical vibration energy with an external pressure force to adhere the wire to the die and leadframe surfaces. However, heat generated during the process may aggravate the oxidation of the leadframe surface, leading to non-stick or unreliability of the bond. Oxidation during the wire bonding process should thus be stopped or at least reduced.
A typical wire bonder uses a window clamp, which is usually rectangular in design, to clamp a leadframe securely to a top plate of a workholder. An industry practice for protecting leadframes from oxidation is to introduce large amounts of a relatively inert gas, usually nitrogen gas, to the leadframe. Various apparatus have been used to do this. In a typical apparatus, an area of the leadframe is covered by the body of the window clamp and is thus relatively well-protected from oxidation, leaving an opening inside the body of the window clamp which leads to a bonding area. The bonding area is exposed to the atmosphere, and is most vulnerable to oxidation.
One method of introducing an inert gas is to locate one or more nozzles next to the bonding area to blow the inert gas into the bonding area. The inert gas in the environment around the bonding area would tend to inhibit oxidation reaction of the leadframe at the high bonding temperatures. In another method, the inert gas can be introduced via channels in the window clamp. U.S. Pat. No. 7,182,793 entitled “System for Reducing Oxidation of Electronic Devices” discloses an example of such a method. An opening of the window clamp is positioned over the leadframe to provide access to the bonding area comprising a die and part of the leadframe. The inert gas enters the bonding area from a gas supply outside the window clamp via gas inlets, conduits and a cavity fabricated in the window clamp. The opening is not covered in order for the bondhead to access the die during wire bonding. Since the size of the opening can be quite large, there is a substantial leakage of inert gas into the atmosphere through the opening, in particular when the bonding area is larger. The leadframe is thus more likely to be exposed to oxygen in the general atmosphere, hence promoting increased oxidation of the leadframe.
Japanese Publication No. JP 59-025232 entitled “Bonding Device” discloses a shielding plate for use in wire bonding, where the shielding plate shields an opening section of a cover that is located over a bonding area of a leadframe. The shielding plate forms a bonding work window allowing access to the bonding area by a wire bonding tool. The shielding plate is mounted such that it is rigidly interlocked with an X-Y table which moves the shielding plate together with movement of a bond head holding the wire bonding tool in both the x and y axes. In this way, leakage of an inert gas from under the cover is confined to the bonding work window and therefore kept to a minimum. While the shielding plate allows bonding to be conducted with minimal loss of the inert gas, moving the shielding plate in more than one axis poses a disadvantage as a large working space must be provided for the movement of the shielding plate both in the x and the y axes. It is therefore necessary to look into ways to reduce the size of a wire bonding apparatus by limiting a range of movement of the shielding plate.